People with mobility disabilities are at greater risk than the general population for incurring health problems such as pressure sores, urinary tract infections, high blood pressure, diabetes, and obesity. Many of these conditions are preventable through behavior and lifestyle changes. In particular, exercise and physical activity are widely recognized as having substantial health benefits, and recent evidence suggests that people with disabilities experience the same physiologic response to exercise as the general population. Nonetheless, nearly three-fourths of those with disabilities report being entirely sedentary or not active enough to achieve health benefits-largely due to barriers that are specifically related to the disability as well as those faced by the general population. This proposed randomized-controlled trial will test the effectiveness of a behavioral intervention to promote physical activity adoption and maintenance among people with mobility impairments. One-hundred eighty manual wheelchair users will be randomly assigned to an intensive 6-month, home-based intervention or a minimal-contact control group and will be followed for another 6 months after the intervention ends. In this study, we aim to assess: (1) the effectiveness of the intervention for promoting physical activity adoption and maintenance, (2) the physical and psychosocial effects of the intervention, and (3) the complex interplay of factors that influence the effectiveness of the intervention. It is hypothesized that the experimental group will engage in more minutes of physical activity at 12 weeks, maintain higher rates of physical activity at 26 and 52 weeks, have larger improvements in strength and aerobic capacity, have larger improvements in perceived function and participation, and have larger improvements in exercise self-efficacy, well being, quality of life, and depressive symptoms than the control group. In addition, various hypotheses will examine the way in which barriers to exercise, exercise self-efficacy, and disability-related exacerbations operate; that is whether they have simple additive effects or operate in more complex ways as or with mediators (i.e., have indirect effects) or as moderators (i.e., have interactive effects). Data collected from this study will yield unique and important evidence regarding strategies to promote physical activity among manual wheelchair users, as well as specific effects of physical activity on the physical and psychosocial health, function, and participation of community-dwelling manual wheelchair users.